Accepted Answer

It means you need to press 1 in the air after you've performed a running jump. To clarify...

1. Dash on the ground (tap left or right twice and hold)2. Perform a jump during your dash (press 2 while still holding left or right from your dash)3. Press the attack button while in midair following this jump (press 1 after jumping, but before you land on the ground)

Test this out with Sword. You should do the spinning attack in midair if you did it right. The neutral jump attack is a simple midair chop.

Other Answers

Also, not every ability has a dashing jump attack. Off the top of my head, those are Sword, Beam, Fire (same as the ground dash attack), Ice (faster version of the neutral jump attack) and Hammer (same as Ice). There are probably more, but I forget.